An important trend in development of semiconductor technology is scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) for improving integration level and reducing manufacturing cost. However, it is well known that short channel effects arise as the size of MOSFETs decreases. As the MOSFETs are scaled down, a gate also has a reduced effective length and actually controls fewer charges in a depletion region when a gate voltage is applied. Consequently, a threshold voltage of the MOSFETs drops with a reduced channel length.
In the MOSFETs, it may be desirable on the one hand that the threshold voltage of the semiconductor device is increased to suppress the short channel effects, and on the other hand that the threshold voltage of the semiconductor device is decreased to reduce power consumption in a low supply voltage application, or in an application using both P-type and N-type MOSFETs.
Channel doping is a known approach of tuning the threshold voltage. However, if the threshold voltage of the semiconductor device is raised by increasing a doping concentration in a channel region, mobility of carriers drops, which results in degradation of the device performance. Moreover, ions with a high doping concentration in the channel region may neutralize ions in source/drain regions and ions in regions which adjoin the channel region, which decreases a doping concentration in the region adjacent to the channel region and increases resistance of the device.
The short channel effects can be suppressed by providing a ground plane (i.e. a grounded back gate) beneath a buried insulating layer. However, an integrated circuit may comprise MOSFETs with different gate lengths. Although a high doping concentration in a back gate may be beneficial for the MOSFET with a short gate length for suppressing the short channel effects, it causes an excessively high threshold voltage for the MOSFET with a long gate length. It is desirable that the threshold voltage is adjusted differently for the MOSFETs with different gate lengths.
Therefore, it is still desirable that the threshold voltage of the semiconductor device is adjusted in a controllable manner without increasing the doping concentration in the channel region, while the performance of the semiconductor device is not deteriorated.